Conventionally, a piezoelectric body has been used with a large electromechanical coupling factor such as a lithium niobate (LiNbO3) substrate to achieve an acoustic wave filter with wide-band characteristics.
However, an acoustic wave filter using this type of piezoelectric body typically has a disadvantage of poor temperature characteristics. To improve temperature characteristics, a way is devised in which a dielectric thin film made of SiO2 is formed on a piezoelectric body made of lithium niobate.
FIGS. 31A and 31B show a configuration of a conventional acoustic wave resonator. FIG. 31A is a top view of the resonator, and FIG. 31B is a sectional view of the resonator, taken along line 31B-31B in FIG. 31A.
In FIGS. 31A and 31B, conventional acoustic wave resonator 101 includes piezoelectric body 102, and IDT (interdigital transducer) electrode 103 provided on piezoelectric body 102, for exciting an acoustic wave with wavelength λ. Resonator 101 includes dielectric thin film 105 provided on piezoelectric body 102 so as to cover IDT electrode 103. Electrode 103 includes bus bar electrode region 106, dummy electrode region 107, and IDT cross region 108, in order from outside.
Acoustic wave resonator 101 further has opening 109 in dielectric thin film 105 above bus bar electrode region 106 and dummy electrode region 107 to expose IDT electrode 103 in the regions.
This structure enables the sound velocity of an acoustic wave in bus bar electrode region 106 and dummy electrode region 107 of acoustic wave resonator 101 to be faster than that in IDT cross region 108. This condition, prevents leakage of an acoustic wave from IDT cross region 108 to dummy electrode region 107, which reduces insertion degradation loss of the acoustic wave.
As shown in FIG. 32, however, conventional acoustic wave resonator 101 disadvantageously generates more transverse-mode spurious emission between the resonance frequency and the antiresonance frequency. This is because conventional acoustic wave resonator 101 confines transverse-mode waves as well as main acoustic waves into IDT cross region 108.
There is known patent literature 1 as a prior art document related to the patent application.